X-ray intensifying screen including a titanium activated hafnium dioxide phosphor containing lutetium to reduce afterglow

ABSTRACT

An intensifying screen is disclosed containing a phosphor composition comprised of monoclinic crystals of a titanium activated hafnia phosphor host containing lutetium ions to reduce afterglow. A phosphor composition in which zirconia is at least partially substituted for hafnia is also disclosed.

RELATED APPLICATIONS

This is a continuation-in-part of U.S. Ser. No. 305,222, filed Feb. 3,1989.

FIELD OF THE INVENTION

The invention relates to novel X-ray intensifying screens. Morespecifically, the invention relates to fluorescent screens of the typeused to absorb an image pattern of X-radiation and to emit acorresponding pattern of longer wavelength electromagnetic radiation.The invention additionally relates to certain novel phosphorcompositions and to processes for their preparation.

BACKGROUND OF THE INVENTION

A developable latent image is formed in a silver halide emulsion layerof a radiographic element when it is imagewise exposed to X-radiation.Silver halide emulsions, however, more efficiently absorb andconsequently are more responsive to longer (300 to 1500 nm) wavelengthelectromagnetic radiation than to X-radiation. Silver halide possessesnative sensitivity to both the near ultraviolet and blue regions of thespectrum and can be sensitized readily to the green, red, and infraredportions of the electromagnetic spectrum.

Consequently it is an accepted practice to employ intensifying screensin combination with silver halide radiographic elements. An intensifyingscreen contains on a support a phosphor layer that absorbs theX-radiation more efficiently than silver halide and emits to theadjacent silver halide emulsion layer of the radiographic element longerwavelength electromagnetic radiation in an image pattern correspondingto that of the X-radiation received.

The most common arrangement for X-radiation exposure is to employ a dualcoated radiographic element (an element with silver halide emulsionlayers on opposite sides of a support), each emulsion layer beingmounted adjacent a separate intensifying screen. The radiographicelement is a consumable, used to record a single imagewise exposure,while the intensifying screens are used repeatedly.

If the luminescence of an intensifying screen persists after imagewiseexposure to X-radiation has been terminated, there is a risk that theafterglow will expose the next radiographic element brought into contactwith the screen. Thus, the measure of a satisfactory intensifying screenis not only the intensity of the luminescence it exhibits upon exposureto X-radiation, but also the rapidity with which the luminescence decaysupon the termination of X-radiation exposure.

Of the many different phosphor compositions known, most have failed tosatisfy the practical demands of intensifying screen application forfailing to generate sufficient emission intensity upon exposure toX-radiation, for exhibiting persistent luminescence after exposure(afterglow), or a combination of both.

Phosphors employed in intensifying screens consist of a host compound,often combined with a small amount of another element that changes thehue and/or improves the efficiency of fluorescence. It has beenrecognized that useful phosphors are those in which the host compoundcontains at least one higher atomic number element to facilitateabsorption of the high energy X-radiation. For example, barium sulfate,lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calciumtungstate, are widely employed phosphor host compounds.

From time to time various compounds of zirconium and hafnium have beeninvestigated as phosphors. Zirconium and hafnium are known to be atomsof essentially similar radii, 1.454 Å and 1.442 Å, respectively.Practically all known compounds of zirconium and hafnium correspond tothe +4 oxidation state. The chemical properties of the two elements areessentially identical.

Hale U.S. Pat. No. 2,314,699, issued Mar. 23, 1943, discloses a methodof preparing a luminescent material which comprises dispersing an oxideof an element chosen from the group consisting of beryllium, magnesium,zinc, and zirconium in a solution of a salt of an element chosen fromthe group consisting of silicon, germanium, titanium, zirconium,hafnium, and thorium, and precipitating the dioxide of the element ofthe second named group upon the oxide of the element of first namedgroup.

Leverenz U.S. Pat. No. 2,402,760, issued June 25, 1946, discloses acrystalline luminescent material represented by the general formula:

    u(BeO)v(XO.sub.2)w(YO.sub.2):xMn

where X is a metal selected from the group of metals consisting ofzirconium, titanium, hafnium, and thorium, Y is an element selected fromthe group of elements consisting of silicon and germanium, the molarratio

    u/v

being from 1/99 to 99, the molar ratio of

    u+v/w

being from 1/3 to 2, and the sum of u+v being equal to one grammolecular weight.

Zirconium and hafnium containing compounds also containing rare earthelements have also been disclosed from time to time:

Anderson U.S. Pat. No. 3,640,887, issued Feb. 8, 1972, disclosestransparent polycrystalline ceramic bodies composed of oxides ofthorium, zirconium, hafnium, and mixtures thereof with oxides of therare earth elements 58 through 71 of the Periodic Table optionallyadditionally including yttria. Anderson contains no mention ofluminescence.

Mathers U.S. Pat. No. 3,905,912, issued Sept. 16, 1975, discloses ahafnium phosphate host phosphor with an activator selected from amongterbium, praseodymium, dysprosium, thulium, and europium.

Kelsey, Jr. U.S. Pat. No. 4,006,097, issued Feb. 1, 1977, disclosesytterbium activated hafnia phosphors.

Chenot et al U.S. Pat. No. 4,068,128, issued Jan. 10, 1978, discloses asa phosphor for luminescent intensifying screens (Hf_(1-x) Zr_(x))O₂ :P₂O₅, where x is in the range of from 0 to 0.5. Eu⁺² is disclosed toenhance blue emission.

Chenot et al U.S. Pat. No. 4,112,194, issued Sept. 5, 1978, discloses asa phosphor for luminescent intensifying screens (Hf_(1-x) Zr_(x))_(3-y)A_(4y) (PO₄)₄, where x is within the range of about 0.005 to 0.5, A isselected from the group consisting of lithium, sodium, and potassium,and y is within the range of 0.4 to 2.0. Eu⁺² is disclosed as anactivator for a green emitting phosphor.

Alexandrov et al U.S. Pat. No. 4,153,469, issued May 8, 1979, disclosesas artificial precious stones or laser elements monocrystals ofzirconium or hafnium oxide stabilized with yttrium oxide.

Klein et al U.S. Pat. No. 4,295,989, issued Oct. 20, 1981, discloses acubic yttria stabilized hafnia phosphor doped with Ce³⁺.

E. Iwase and S. Nishiyama, "Luminescence Spectra of Trivalent Rare EarthIons", Proc. Intern. Sym. Mol. Struct. Spectry., Tokyo, 1962, A-407-1 to7, report the crystal lattice constants of monoclinic hafnia andzirconia as follows:

                  TABLE I                                                         ______________________________________                                        Oxide     a-axis  b-axis      c-axis                                                                              β                                    ______________________________________                                        HfO.sub.2 5.11    5.14        5.28  99° 44'                            ZrO.sub.2 5.21    5.26        5.375 99° 55'                            ______________________________________                                    

Iwase and Nishiyama investigated hafnia and zirconia forcathodoluminescence--i.e., fluorescence response to electronbombardment. The emission characteristics of these oxides doped withtrivalent samarium, praseodymium, dysprosium, terbium, and europium ionsare reported.

It has been recognized that the inclusion of titanium as an activatorcan significantly increase the luminescence of zirconia and hafnia:

Kroger U.S. Pat. No. 2,542,336, issued Feb. 20, 1951, discloses aphosphor containing titanium as an activator and having a matrixcomposed of one or more of the oxides of zirconium, hafnium, thorium,germanium or tin, to which may be added either acid oxides or basicoxides or both.

L. H. Brixner, "Structural and Luminescent Properties of the Ln₂ Hf₂ O₇-type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984,describes investigations of title phosphor host compounds. Ln is definedto include not only lanthanides, but also scandium and yttrium. Afterreporting the properties of Ti⁺⁴ as an activator for rare earthhafnates, Brixner states:

We also looked at this same activator in pure HfO₂. Under 30 kVp Moradiation x-ray excitation, this composition also emits in a broad bandcentered around 477 nm as seen in FIG. 5. This emission has an intensityof about 1.6 times that of PAR CaWO₄ and could therefore be of interestas an x-ray intensifying screen phosphor, especially in light of thesuperior absorption of HfO relative to CaWO as seen in FIG. 6.Unfortunately, the price of optical grade HfO is so prohibitive that itcannot be used in screen applications. (Emphasis added.)

J. F. Sarver, "Preparation and Luminescent Properties of Ti-ActivatedZirconia", Journal of the Electrochemical Society, Vol. 113, No. 2, Feb.1966, pp. 124-128, discloses investigations of Ti⁺⁴ activation ofzirconia. Sarver states:

At room temperature the phosphor exhibits a very rapid initialexponential decay . . . similar to CaWO₄ and MgWO₄ and some sulfidephosphors . . . Beyond about 20 μsec, the decay rate becomes much slowerand the phosphorescence is visually detectable for a few minutes. It wasfound that the addition of certain mineralizers or fluxes, in particular1 mole % LiF, besides leading to an expected increase in particle sizeduring firing, also causes an increase in the intensity of thephosphorescence although the intensity of the fluorescence is virtuallythe same . . .

RELATED PATENT APPLICATION

Bryan et al U.S. Ser. No. 305,310, filed Feb. 2, 1989, now abandoned infavor of U.S. Ser. No. 393,602, filed Aug. 14, 1989, commonly assigned,titled PHOSPHOR COMPOSITION AND X-RAY INTENSIFYING SCREEN, discloses thepreparation of lithium hafnate phosphors. The phosphor crystals aredisclosed to consist essentially of oxygen and a combination of metalssatisfying the relationship:

    Li.sub.2 Hf.sub.1-x-y-z Zr.sub.z Sn.sub.y Ti.sub.x L.sub.w

where

L is a rare earth, where "rare earth" is herein defined to includelanthanides, yttrium, and scandium;

w+x+y are together 0 to 0.2; and

z is up to 0.2.

SUMMARY OF THE INVENTION

In one aspect, this invention is directed to a screen comprised of asupport and a fluorescent layer containing a phosphor capable ofabsorbing X-radiation and emitting longer wavelength electromagneticradiation comprised of monoclinic crystals of a titanium activatedhafnia phosphor host. The intensifying screen is characterized in thatlutetium is present in the monoclinic crystals in an amount sufficientto reduce afterglow.

In another aspect this invention is directed to monoclinic crystals of aphosphor host consisting essentially of at least one of zirconia andhafnia containing an amount of titanium ions sufficient to increaseluminescence intensity during exposure to stimulating radiation and anamount of lutetium ions sufficient to reduce the intensity ofluminescence persisting after exposure to stimulating radiation.

DESCRIPTION OF PREFERRED EMBODIMENTS

An essential and novel feature of the present invention is the discoverythat the addition of lutetium ions to a phosphor host consistingessentially of at least one of zirconia and hafnia containing an amountof titanium ions sufficient to increase luminescence intensity duringexposure to stimulating radiation can overcome the disadvantage ofphosphorescence (alternatively referred to as persistent luminescence orafterglow) associated with titania activated zirconia and hafniaphosphors. By reducing afterglow the invention makes titanium activatedzirconia and hafnia phosphors available for applications requiringprompt decay of emission upon the cessation of external stimulation.

Any form of radiation can be employed known to stimulate zirconia orhafnia phosphors--e.g., X-radiation, ultraviolet radiation, or cathoderays. Since the more energetic forms of radiation require a higheratomic mass for efficient absorption, it is specifically preferred toemploy a hafnia phosphor host when X-radiation is employed forstimulation.

In a specific, preferred form the invention is directed to anintensifying screen comprised of a support and a fluorescent layercontaining a phosphor capable of absorbing X-radiation and emittinglonger wavelength electromagnetic radiation comprised of monocliniccrystals of a titanium activated hafnia phosphor host. Lutetium ispresent in the monoclinic crystals in an amount sufficient to reduceafterglow.

By reducing afterglow it is possible for the first time to employ atitanium activated hafnia phosphor host in an X-ray intensifying screenintended to expose silver halide radiographic elements in rapidsuccession. The afterglow reducing effect of lutetium reduces the riskthat a radiographic element mounted adjacent the X-ray intensifyingscreen will receive an imagewise exposure from the X-ray intensifyingscreen attributable to emission persistence from a previous X-radiationexposure.

Since the chemical similarities of zirconium and hafnium atoms preventtheir complete separation, it is appreciated that even the purestattainable forms of zirconia also contain at some residual hafnia andvice versa. The phosphor compositions of this invention are contemplatedto include as a phosphor host the full range of possible zirconia tohafnia ratios.

The hafnia phosphor hosts contemplated for use in X-ray intensifyingscreens are contemplated to satisfy the relationship:

    Hf.sub.1-z Zr.sub.z                                        (I)

where z is up to 0.3. Optical grade hafnia, the purest form of hafniareadily commercially attainable, contains less than about 3×10⁻⁴ mole ofzirconia per mole of hafnia. Contrary to what has heretofore suggestedby the art, when the zirconia content of the hafnia phosphor host isincreased above the levels found in optical grade hafnia an increase inluminescence is observed. Preferred phosphors are therefore those inwhich z is in the range of from 4×10⁻⁴ to 0.3, most preferably from1×10⁻³ to 0.2, and optimally from 2×10⁻³ to 0.1. The practicalsignificance of this discovery is that reagent grade hafnia,commercially available with z being slightly less than 2×10⁻², can beemployed as a hafnia phosphor host.

The small amounts of other elements found in commercially availablereagent grade hafnium and zirconium source compounds are not detrimentalto intensifying screen performance. Therefore, other possible impuritiesof the phosphor host need be given no further consideration.

In the simplest form of the invention monoclinic reagent grade hafnia orzirconia can be purchased and formed into a phosphor satisfying therequirements of this invention. To form monoclinic phosphor particlescontaining a selected ratio of hafnium and zirconium, commerciallyavailable sources of zirconium and hafnium are intimately intermixed,preferably by being dissolved in a common solvent, followed bycoprecipitation. The hafnium and zirconium containing mixture is chosenso that upon firing only hafnium, zirconium, and oxygen atoms remain asresidue, any other moieties of the compounds being thermally decomposedor otherwise driven off in firing.

Common sources of hafnium and zirconium include the dioxides, the basiccarbonates, the oxychlorides, the oxynitrates, the sulfates, and thetetrachlorides. While the dioxides, the basic carbonates, and thesulfates can be used as purchased to produce phosphors, it isadvantageous for both handling and phosphor performance to convert theother sources to less soluble solids that can be fired to give themonoclinic DO₂ phosphor desired, where D represents zirconium orhafnium. For example, treatment of aqueous hafnium and zirconium ioncontaining solutions with base (e.g., alkali or ammonium hydroxide)gives a precipitate which is a mixture of hydrous hafnia and hydrouszirconia, the relative proportions of which depend upon those present inthe starting materials.

Other useful solids satisfying phosphor host requirements can beproduced by treating hafnium and zirconium ion containing solutions withorganic precipitating agents, since organic materials consisting ofcarbon, hydrogen, and optionally nitrogen and/or oxygen leave noobjectionable residue upon thermal decomposition.

Hafnium and zirconium can be conveniently coprecipitated ascarboxylates, such as those containing from about 2 to 20 carbon atoms.The carboxylate moieties are in one preferred form aliphaticcarboxylates containing from about 2 to 10 carbon atoms, including bothmonocarboxylates and polycarboxylates--particularly dicarboxylates, suchas oxalates, succinates, fumarates, etc. Aromatic carboxylates, such asbenzoates, phthalates, and their ring substituted homologues, are alsoconvenient to use. A particularly preferred class of carboxylates areα-hydroxycarboxylates containing from 2 to 10 carbon atoms, such asglycolates, lactates, and mandelates. Oxalic acid can be viewed aseither a dicarboxylic acid or an α-hydroxycarboxylic acid. Oxalates areparticularly preferred moieties for forming not only hafnium andzirconium compounds, but also compounds of other metals to beincorporated in forming preferred forms of the phosphor moreparticularly described below. The carboxylate moieties can form simplecarboxylates with the hafnium or zironium or can form hafnium orzirconium carboxylate complexes including additional cations, such asalkali metal or ammonium ions.

The hafnium and zirconium carboxylates can be conveniently formed byreacting in a common solvent the acid, salt, or ester of the carboxylatewith hafnium and zirconium containing compounds in the ratios desired inthe phosphor. The hafnium and zirconium containing compounds to bereacted can be selected from among compounds such as hafniumtetrachloride, zirconium tetrachloride, hafnium oxychloride, zirconiumoxychloride, hafnium basic carbonate, zirconium basic carbonate, hafniumnitrate, zirconium nitrate, zirconium carbonate, hafnium sulfate,zirconium sulfate, and mixtures thereof.

It is also contemplated to employ hafnium and zirconium alkoxides asstarting materials. Preferred hafnium and zirconium alkoxides are thosewhich satisfy formula II:

    D(OR).sub.4                                                (II)

where

D represents zirconium or hafnium and

R represents a hydrocarbon moiety containing from about 1 to 20(preferably about 1 to 10) carbon atoms. The hydrocarbon moieties can bechosen from any convenient straight or branched chain or cyclicsaturated or unsaturated aliphatic hydrocarbon moiety--e.g., alkyl,cycloalkyl, alkenyl, or alkynyl. Alternatively the hydrocarbon moietycan be an aromatic moiety--e.g., benzyl, phenyl, tolyl, xylyl, naphthyl,etc. In a specifically preferred from R is in each instance lower alkylof from 1 to 4 carbon atoms. Hafnium and zirconium alkoxides aredisclosed in U.S. Pat. Nos. 3,297,414; 3,754,011; 4,525,468, and4,670,472, the disclosures of which are here incorporated by reference.

In addition to alkoxide and carboxylate moiety containing hafnium andzirconium compounds various chelates, such as hafnium and zirconiumβ-diketones and diaminecarboxylates can be employed. Exemplary usefulhafnium starting materials are set forth under heading III below. Allthe compounds have otherwise identical zirconium analogues. Further,although water of hydration has been omitted, it is to be understoodthat under normal ambient conditions most of the compounds exist ashydrates.

    ______________________________________                                        (III)                                                                         Exemplary Hafnium Starting Materials                                          ______________________________________                                        H-1     Hafnyl oxalate                                                                HfO(C.sub.2 O.sub.4)                                                  H-2     Hafnyl oxalic acid                                                            H.sub.2 [HfO(C.sub.2 O.sub.4).sub.2 ]                                 H-3     Dioxalatohafnium                                                              Hf(C.sub.2 O.sub.4).sub.2                                             H-4     Trioxalatohafnic acid                                                         H.sub.2 [Hf(C.sub.2 O.sub.4).sub.3 ]                                  H-5     Ammonium trioxalatohafnate                                                    (NH.sub.4).sub.2 [Hf(C.sub.2 O.sub.4).sub.3 ]                         H-6     Potassium tetraoxalatohafnate                                                 K.sub.4 [Hf(C.sub.2 O.sub.4).sub.4 ]                                  H-7     Sodium tetraoxalatohafnate                                                    Na.sub.4 [Hf(C.sub.2 O.sub.4).sub.4 ]                                 H-8     Ammonium hafnyl oxalate                                                       (NH.sub.4).sub.2 [HfO(C.sub.2 O.sub.4).sub.2 ]                        H-9     Polyoxalatopolyhafnic acids                                           H-10    Potassium hafnyl tartrate                                                     K.sub.2 [HfO(C.sub.4 H.sub.4 O.sub.6).sub.2 ]                         H-11    Tetramandelatohafnic acid                                                     H.sub.4 [Hf(O.sub.2 CCHOC.sub.6 H.sub.5).sub.4 ]                      H-12    Triglycolatohafnic acid                                                       H.sub.3 HfOH(OCH.sub.2 COO).sub.3                                     H-13    Trilactohafnic acid                                                           H.sub.3 HfOH(OCHCH.sub.3 COO).sub.3                                   H-14    Trioxodihafnium stearate                                                      Hf.sub.2 O.sub.3 (O.sub.2 C(CH.sub.2).sub.16 CH.sub.3).sub.2          H-15    Trioxodihafnium 2-ethylcaproate                                               Hf.sub.2 O.sub.3 (O.sub.2 CCHC.sub.2 H.sub.5 (CH.sub.2).sub.3                 CH.sub.3).sub.2                                                       H-16    Hafnium acetylacetonate                                                       Hf(C.sub.5 H.sub.7 O.sub.2).sub.4                                     H-17    Potassium bisnitrilotriacetohafnate                                           K.sub.2 {Hf[N(CH.sub.2 CO.sub.2).sub.3 ]}                             H-18    Hafnium ethylenediaminetetraacetic acid                                       Hf[(O.sub.2 CCH.sub.2).sub.2 NCH.sub.2 ].sub.2                        H-19    Hafnyl malonate                                                               HfO(O.sub.2 CCH.sub.2 CO.sub.2)                                       H-20    Hafnyl phthalate                                                              HfO(O.sub.2 C.sub.6 H.sub.4 CO.sub.2)                                 H-21    Hafnium tetraisopropoxide                                                     Hf(OC.sub.3 H.sub.7).sub.4                                            H-22    Hafnium tetra- .sub.- t-amyloxide                                             Hf(OC.sub.5 H.sub.11).sub.4                                           H-23    Hafnium tetra(phenoxide)                                                      Hf(OC.sub.6 H.sub.5).sub.4                                            H-24    Hafnium di(isopropoxide) bis(2-ethoxyethoxide)                                Hf(OC.sub.3 H.sub.7).sub.2 (OC.sub.2 H.sub.4 OC.sub.2 H.sub.5).sub            .2                                                                    H-25    Hafnium tetra(cyclohexoxide)                                                  Hf(OC.sub.6 H.sub.11).sub.4                                           H-26    Hafnium di(isopropoxide) bis[2-(2-n-dodecan-                                  oxyethoxy)ethoxide]                                                           Hf(OC.sub.3 H.sub.7).sub.2 (OC.sub.2 H.sub.4 OC.sub.2 H.sub.4                 OC.sub.12 H.sub.25).sub.2                                             ______________________________________                                    

Formation of the monoclinic phosphor host is achieved by heating thezirconium and/or hafnium compounds to temperatures up to and including1400° C. Higher firing temperatures can, of course, be undertaken, sincethe phosphor possesses high thermal stability. However, it is a distinctadvantage of this invention that firing temperatures above 1400° C. arenot required. Preferred firing temperatures are in the range of fromabout 900° to 1300° C.

Firing is continued until conversion to the monoclinic phase isachieved. For maximum firing temperatures the duration of firing can beless than 1 hour. While extended firing times are possible, once thephosphor has been converted to the monoclinic crystalline form,extending the duration of firing serves no useful purpose. Generallyfiring times in the range of from 1 to 10 hours more typically 2 to 5hours, provide full conversions of the starting materials to thephosphor composition sought.

Since the starting materials are in most instances decomposed attemperatures well below the 900° C. minimum temperature levelcontemplated for monoclinic crystal growth, it is generally convenientto heat the starting materials to a temperature above theirdecomposition temperature, but below 900° C., for an initial period topurge volatilizable materials before progressing to the highercrystallization temperatures. Typically, a preliminary heating step inthe range of from about 300° to 900° C., preferably in the range of from400° to 700° C., is undertaken.

It is also often convenient to divide firing into two or moreconsecutive steps with intermediate cooling to permit grinding and/orwashing the material. Intermediate grinding can facilitate uniformitywhile intermediate washing, typically with distilled water, reduces therisk of unwanted contaminants, such as starting material decompositionby-products.

It has been discovered that firing the phosphor in the presence of aflux of one or a combination of akali metal ions incorporates alkalimetal ion in the phosphor and dramatically increases its luminescenceintensity. A preferred class of phosphors according to the presentinvention are those that satisfy the relationship:

    DM.sub.y                                                   (IV)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z M.sub.y                                (V)

where

M represents at least one alkali metal;

y is in the range of from 1×10⁻⁴ to 1 (preferably 0.2); and

D and z are as defined above.

Investigations have revealed that the benefits of alkali metal ioninclusion are fully realized at relatively low concentrations andincorporation of alkali metal ions in concentrations above thoserequired for maximum luminescence enhancement are not detrimental toluminescence. There is no phosphor performance basis for limiting y tovalues of 1 or less. Rather it is primarily a phosphor preparationconvenience.

Alkali metal ion inclusion in the phosphor can be convenientlyaccomplished by forming a mixture of the hafnium and/or zirconiumstarting materials discussed above and a compound capable of releasingalkali metal ions on heating. The amount of the alkali metal compoundemployed is chosen to supply alkali metal ion in a concentration inexcess of that following is contemplated as a starting materialrelationship:

    DM.sub.m                                                   (VI)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z M.sub.m                                (VII)

wherein

M represents at least one alkali metal;

m is greater than 3×10⁻² (preferably from 1×10⁻¹ to 6); and

D and z are as defined above.

The alkali metal compounds can be alkali metal analogues of the hafniumand zirconium starting materials discussed above. Preferred alkali metalcompound starting materials include alkali metal carbonates, sulfates,oxalates, halides, hydroxides, borates, tungstates, and molybdates.Mixtures of alkali metal starting materials are contemplated,particularly when different alkali metals are being concurrentlyincorporated in the phosphor. Since in one form the hafnium and/orzirconium complexes of formula II can contain alkali metal ion, thealkali metal can wholly or in part be provided by these complexes. Aconvenient preparation approach is to employ alkali metal containinghafnium and/or zirconium complexes satisfying formula II and to increasethe alkali metal content of the starting materials by adding otheralkali metal compounds, as indicated above.

In relationships VI and VII, m can range of up to 10 or more. Most ofthe excess of alkali metal is removed during phosphor preparation. Whenan excess of alkali metal is incorporated in the phosphor, it ispreferred to divide firing into two or more sequential steps withintermediate grinding and washing to remove soluble alkali metalcompounds. This reduces the level of alkali metal compounds availablefor release during heating in a corrosive volatilized form and alsoreduces the possibility of forming less desirable secondary phases.

Investigation of alkali metal containing phosphors indicates that theyexhibit increased levels of luminescence even after extended washing hasreduced the alkali metal content to very low levels, approachingdetection limits. While it is believed that the alkali metal isincorporated into the monoclinic crystals of the phosphor, this has notbeen conclusively established. It is possible that the alkali metalcontent of the phosphor is at least partially a surface remnant of thealkali metal flux on the surface of the monoclinic crystals during theirformation during firing.

The highest levels of phosphor luminescence have been obtained byemploying lithium as an alkali metal. In a preferred form lithiumcontaining phosphors according to this invention satisfy therelationship:

    DLi.sub.y                                                  (VIII)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z Li.sub.y                               (IX)

wherein

y is in the range of from 8×10⁻⁴ to 0.15 and

D and z are as defined above.

Lithium containing phosphors according to this invention are preferablyprepared by selecting starting materials so that the hafnium, zirconium,and lithium ions present prior to heating satisfy the followingrelationship:

    DLi.sub.m                                                  (X)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z Li.sub.m                               (XI)

wherein

m is in the range of from 4×10⁻² to 2.0 (optimally from 7×10⁻² to 1.5)and

D and z are as defined above.

When lithium is selected as the alkali metal, it has been observed that,in addition to forming a hafnia phosphor host with lithium included, asecond phase of lithium hafnate can be formed, depending upon theproportion and selection of lithium compound starting materials. Sincelithium hafnate lacks the luminescence intensities of titanium andlithium activated hafnia, a preferred embodiment of the invention,lithium starting materials and their concentrations are selected so thatany overall luminesence of the two phases remains higher than thatattained in the absence of lithium. Increasing levels of lithiumcarbonate employed as a starting material results first in an increasein overall luminescence eventually followed by a decrease in overallluminescence attributed to the formation of increasingly largerproportions of lithium hafnate. On the other hand, employing lithiumsulfate as a starting material, increasing proportions result in peakluminescence with still higher proportions of lithium sulfate resultingin a relatively constant high level of luminescence, indicating that theproportion of lithium hafnate which is formed as a second phase islimited at higher lithium sulfate concentrations in the startingmaterials.

Sodium and potassium compounds employed as starting materials in placeof lithium compounds also result in markedly increased levels ofphosphor luminescence. These alkali metal starting materials, of course,avoid any possibility of forming a lithium hafnate second phase and cantherefore be employed well above the preferred maximum concentrationlevels of lithium starting materials without any performance penalty. Onthe other hand, it has been observed that sodium and potassium ions arequite effective at lower concentrations. Therefore, when M inrelationships IV and V represents at least one of sodium and potassium,y is preferably in the range of from 6×10⁻⁴ to 7×10⁻² (optimally from8×10⁻⁴ to 7×10⁻²).

The alkali metals cesium and rubidium are also effective to increasephosphor luminescence, but to a lesser extent than lithium, sodium, andpotassium. Combinations of any and all of the alkali metals can beemployed in preparing the phosphors of this invention. Particularlyuseful are combinations of at least two of lithium, sodium, andpotassium ions. Lithium and potassium ion combinations have producedparticularly high levels of luminescence.

The fluorescence efficiencies of the phosphors of this invention areincreased by blending with the phosphor host before firing a smallamount of a titanium activator. Titanium activation can be undertakenaccording to any conventional technique, such as any of the techniquesdescribed by Kroger, Brixner, and Sarver, cited above and hereincorporated by reference. Hafnium, zirconium, and titanium are presentand satisfy the relationship

    DTi.sub.x                                                  (XII)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z Ti.sub.x                               (XIII)

wherein

x is the range of from 3×10⁻⁴ to 1.0 (preferably 0.5 and optimally 0.25)and

D and z are as defined above.

It is possible to introduce the titanium activator by physically mixingtitania with any of the host phosphor forming materials described above.It has been discovered, however, that higher luminescence levels atlower titanium concentrations are possible when the titanium activatorin the form of a thermally decomposable compound is physically blendedwith thermally decomposable hafnium and/or zirconium compounds. Thethermally decomposable moieties of the titanium activator compounds canbe selected from among the same compound classes described in connectionwith hafnium and zirconium. Titanium carboxylates, where thecarboxylates are chosen as described above, are particularly preferredstarting materials for the incorporation of titanium.

The inclusion of titanium in the host phosphor not only greatlyincreases the total luminescence of the phosphor, but also shifts themaximum emission wavelength of the phosphor from the ultraviolet to theblue portion of the spectrum. Emissions in the blue portion of thespectrum are more useful for intensifying screen use, since the silverhalide emulsions of radiographic elements which are employed incombination with intensifying screens possess native blue sensitivityand/or can be readily spectrally sensitized to these wavelengths whilethe organic vehicle of the emulsion is transparent in the blue portionof the spectrum.

In a specifically preferred form of the invention the zirconium richhafnia phosphors include both alkali metal ion and titanium, eachintroduced as described above. In this form the phosphor satisfies therelationship:

    DM.sub.y Ti.sub.x

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z M.sub.y Ti.sub.x                       (XV)

wherein

D, M, x, y, and z are as previously defined.

It has been surprisingly discovered that disproportionately largeenhancements of luminescence are realized when both alkali metal ion andtitanium are incorporated in the phosphor. That is, the luminescenceincreases imparted by each of the alkali metal ion and titanium alonewhen added together do not equal or even approach the magnitude of theluminescence increase imparted by a combination of alkali metal ion andtitanium employed together in the phosphor.

To reduce the persistence of luminescence following stimulation (i.e.,phosphorescence or afterglow) a small amount of lutetium is incorporatedin the phosphor host as a dopant. The lutetium can be employed in anyconvenient amount effective to reduce afterglow and can be introduced byany convenient conventional technique for adding rare earth activators.For example, the techniques described above for titanium incorporationalso permit lutetium incorporation in the phosphor host. A preferredtechnique for lutetium introduction is to mix a water soluble lutetiumsalt, such as lutetium nitrate, in solution or to mix a salt or oxide oflutetium in finely divided form with one of the zirconium and/or hafniumstarting materials during or prior to titanium introduction. Thelutetium is distributed within the phosphor host as a dopant duringfiring.

The phosphor in its completed form consists essentially of oxygen andcombined elements satisfying one of the following relationships:

    ______________________________________                                        (XVI)             DTi.sub.x Lu.sub.w                                          (XVII)            Hf.sub.1-z Zr.sub.z Ti.sub.x Lu.sub.w                       (XVIII)           DM.sub.y Ti.sub.x Lu.sub.w                                  and                                                                           (XIX)             Hf.sub.l-z Zr.sub.z M.sub.y Ti.sub.x Lu.sub.w               ______________________________________                                    

wherein

w is in the range of from 1×10⁻⁵ to 1×10⁻³, preferably 5×10⁻⁵ to 3×10⁻⁴and

D, M, w, y, and z are as previously defined.

The phosphors of this invention, once formed, can be employed to serveany conventional use for hafnia and/or zirconia phosphors. Aspecifically preferred application for the phosphors when z is 0.3 orless (i.e., in hafnia phosphor host formulations) is in X-rayintensifying screens. Aside from the inclusion of a phosphor satisfyingthe requirements of this invention, the intensifying screen can be ofany otherwise conventional type. In its preferred construction theintensifying screen is comprised of a support onto which is coated afluorescent layer containing the phosphor of this invention inparticulate form and a binder for the phosphor particles. The phosphorscan be used in the fluorescent layer in any conventional particle sizerange and distribution. It is generally appreciated that sharper imagesare realized with smaller mean particle sizes. Preferred mean particlesizes for the zirconium rich hafnia phosphors of this invention are inthe range of from 0.5 μm to 40 μm, optimally from 1 μm to 20 μm.

It is, of course, recognized that the phosphor particles can be blendedwith other, conventional phosphor particles, if desired, to form anintensifying screen having optimum properties for a specificapplication. Intensifying screen constructions containing more than onephosphor containing layer are also possible, with the phosphor particlesof this invention being present in one or more of the phosphorcontaining layers.

The fluorescent layer contains sufficient binder to give the layerstructural coherence. The binders employed in the fluorescent layers canbe identical to those conventionally employed in fluorescent screens.Such binders are generally chosen from organic polymers which aretransparent to X-radiation and emitted radiation, such as sodiumo-sulfobenzaldehyde acetal of poly(vinyl alcohol); chlorosulfonatedpoly(ethylene); a mixture of macromolecular bisphenol poly(carbonates)and copolymers comprising bisphenol carbonates and poly(alkyleneoxides); aqueous ethanol soluble nylons; poly(alkyl acrylates andmethacrylates) and copolymers of alkyl acrylates and methacrylates withacrylic and methacrylic acid; poly(vinyl butyral); and poly(urethane)elastomers. These and other useful binders are disclosed in U.S. Pat.Nos. 2,502,529; 2,887,379; 3,617,285; 3,300,310; 3,300,311; and3,743,833; and in Research Disclosure, Vol. 154, February 1977, Item15444, and Vol. 182, June 1979. Particularly preferred intensifyingscreen binders are poly(urethanes), such as those commercially availableunder the trademark Estane from Goodrich Chemical Co., the trademarkPermuthane from the Permuthane Division of ICI, Ltd., and the trademarkCargill from Cargill, Inc.

The support onto which the fluorescent layer is coated can be of anyconventional type. Most commonly, the support is a film support. Forhighest levels of image sharpness the support is typically chosen to beblack or transparent and mounted in a cassette for exposure with a blackbacking. For the highest attainable speeds a white support, such as atitania or barium sulfate loaded or coated support is employed.Specifically preferred reflective supports offering the highestattainable balance of speed and sharpness are those containingreflective microlenslets, disclosed by Roberts et al U.S. Ser. No.243,374, filed Sept. 12, 1988, titled AN X-RAY INTENSIFYING SCREENPERMITTING AN IMPROVED RELATIONSHIP OF IMAGING SPEED AND SHARPNESS,commonly assigned now U.S. Pat. No. 4,912,333 issued Mar. 27, 1990.

Any one or combination of conventional intensifying screen features,such as overcoats, subbing layers, and the like, compatible with thefeatures described above can, of course, be employed. Both conventionalradiographic element and intensifying screen constructions are disclosedin Research Disclosure, Vol. 184, Aug. 1979, Item 18431, the disclosureof which and the patents cited therein are here incorporated byreference. Research Disclosure is published by Kenneth MasonPublications, Ltd., Emsworth, Hampshire P010 7DD, England.

In one specifically preferred form of the invention, illustratingintensifying screens satisfying the requirements of the inventionintended to be employed with a separate silver halide emulsion layercontaining radiographic element, the phosphor of this invention can besubstituted for any of the conventional phosphors employed in either thefront or back intensifying screens of Luckey, Roth et al U.S. Pat. No.4,710,637, the disclosure of which is here incorporated by reference.Similar modification of any of the conventional intensifying screensdisclosed in the following patents is also contemplated: DeBoer et alU.S. Pat. No. 4,637,898; Luckey, Cleare et al U.S. Pat. No. 4,259,588;and Luckey U.S. Pat. No. 4,032,471.

While the phosphors of the invention can be employed for their promptemission following exposure to X-radiation, they can also be employed asstorage phosphors--that is, for their ability to emit electromagneticradiation in a chosen wavelength range after being exposed toX-radiation and then stimulated by exposure to radiation in a thirdspectral region. For example, the phosphors of this invention can beemployed in storage phosphor screens and systems of the type disclosedby Luckey U.S. Pat. No. 3,859,527, the disclosure of which is hereincorporated by reference. When employed in such a system the refractiveindices of the phosphor and binder are preferably approximately matched,as disclosed by DeBoer et al U.S. Pat. No. 4,637,898, also incorporatedby reference.

EXAMPLES

The invention can be better appreciated by reference to the followingspecific examples.

Examples 1-9 Phosphors Containing Varied Ratios of Hafnium and Zirconium(Hf_(1-z) Zr_(z))

The purpose of presenting these investigations is to demonstrate that,by varying the zirconium content in a hafnia host phosphor, enhancedphosphor luminescence intensity is achieved over a limited zirconiumconcentration range in which the zirconium content is higher than thatfound in optical grade hafnium sources, but still only a minorconstituent.

Hafnia phosphor samples containing varied amounts of zirconiumsubstituted for hafnium were prepared by the decomposition of theappropriate trilactohafnic and trilactozirconic acid complexes. Thecomplexes were prepared by the general method described in W. B.Blumenthal, "The Chemical Behavior of Zirconium," VanNostrand,Princeton, N.J., 1958, p 333. The varying Hf:Zr ratios are obtained byusing the appropriate mixtures of zirconium and hafnium oxychlorides inthe precipitation reactions. The oxychlorides were obtained fromTeledyne Wah Chang Albany (located at Albany, Oreg.) and used asreceived. The Hf:Zr ratios in the samples were determined from theanalytical batch analyses provided by the supplier.

The preparation of trilactohafnic acid for Example 1 was carried out inthe following manner: Optical grade (Hf_(1-z) Zr_(z), z=0.000276)hafnium oxychloride (40 g) and ACS reagent lactic acid (44 g) fromEastman Kodak Company were each dissolved in about 120 ml of distilledwater. The hafnium oxychloride solution was added to the lactic acidsolution with rapid stirring to form a precipitate, and the resultingmixture was heated to 80° C. with continued stirring for about 0.5hours. The cooled mixture was filtered, and the collected solid waswashed with distilled water. After drying for 15 hours at 80° C., thesolid weighed 42 g. (for C₉ H₁₆ O₁₀ Hf: theory, C=23.4%, H=3.5%, found,C=22.7%, H=3.5%).

Approximately 13 g of the trilactohafnic acid was placed in a 50 mLalumina crucible, covered with an alumina lid, heated in air to 700° C.for one hour in an ashing furnace, then cooled to room temperature. Thesolid was transferred to a 20 mL alumina crucible, which was coveredwith an alumina lid. The covered 20 mL alumina crucible was placed intoa 50 mL alumina crucible, which was thereafter covered with an aluminalid. The crucible assembly was heated to 1000° C. and maintained at thattemperature for 2.5 hours before cooling to room temperature. Theresulting solid was ground with an agate mortar and pestle to give apowder that was returned to the 20 mL alumina crucible. The 20 mLcrucible was covered with its alumina lid and then heated to 1400° C.and maintained at that temperature for 1.5 hours before cooling to roomtemperature. The resulting solid was ground with an agate mortar andpestle to give a uniform phosphor powder.

The Example 1 phosphor powder sample was made from optical grade hafniumoxychloride and contained the lowest amount of zirconium. The Example 5sample was made from reagent grade (designated by the supplier asReactor Grade Special and subsequently also referred to as R.G.S.)hafnium (Hf_(1-z) Zr_(z), z=0.019) oxychloride. The Example 2, 3, 4A,and 4B samples were made by mixing appropriate amounts of the opticalgrade and reagent grade hafnium oxychlorides. The Example 6 to 9 sampleswere made by mixing appropriate amounts of reagent grade hafnium andzirconium oxychloride to obtain a zirconium content indicated in TableII.

The luminescence response of the phosphor powder was in this and allsubsequent Examples measured by placing the phosphor powder sample inaluminum planchets (2 mm high×24 mm diam) at a coverage of about 1.1g/cm² and exposing to X-radiation. The X-ray response was obtained usinga tungsten target X-ray source in an XRD 6™ generator. The X-ray tubewas operated at 70 kVp and 10 mA, and the X-radiation from the tube wasfiltered through 0.5 mm Cu and 1 mm Al filters before reaching thesample. The luminescent response was measured using an IP-28™photomultiplier tube at 500 V bias. The voltage from the photomultiplierwas measured with a Keithley™ high impedance electrometer and isproportional to the total light output of the sample.

The major luminescence peak of the phosphor samples was centered atabout 280 nm. This value was obtained by taking the prompt emissionspectrum of the powder using the unfiltered X-ray source describedabove. The tube was operated at 70 kVp and 30 mA. The spectrum wasacquired with an Instruments S.A. Model HR 320™ grating spectrographequipped with a Princeton Applied Research Model 1422/01™ intensifiedlinear diode array detector. The data acquisition and processing wascontrolled by a Princeton Applied Research Model 1460 OMA III™ opticalmultichannel analyzer. The spectrum was corrected for the spectralresponse of the detector spectrograph combination.

The relative luminescence intensity of the phosphor powder samples as afunction of their zirconium content is set out in Table II.

                  TABLE II                                                        ______________________________________                                        Hf.sub.l-z Zr.sub.z                                                                                       RELATIVE                                          EXAMPLE NO.   Zr CONTENT (z)                                                                              INTENSITY                                         ______________________________________                                        1 (Control)   0.000276      100                                               2             0.00040       231                                               3             0.0010        238                                               4A            0.01          710                                               4B            0.01          743                                               5             0.019         365                                               6             0.10          350                                               7             0.20          155                                               8             0.30          224                                               9 (Control)   0.50           80                                               ______________________________________                                    

The data of Table II demonstrate that there is an enhancement in hafniaphosphor performance when the zirconium level increased over that foundin optical grade hafnium sources (represented by the Control 1). Rangesof z of from 4×10⁻⁴ (0.0004) to 0.3 are demonstrated to exhibit higherluminescence intensities than optical grade hafnia. Best results aredemonstrated when z is in the range of from 1×10⁻³ (0.001) to 0.2,optimally in the range of from 5×10⁻³ (0.005) to 0.1.

Examples 10-14 Preparation of Phosphors in the Presence of an AlkaliMetal Ion (DM_(m))

The purpose of presenting these investigations is to demonstrate thatthe performance of hafnia host phosphors with an elevated zirconiumlevel shown to be effective in Examples 1-9 can be further dramaticallyimproved by preparing the hafnia phosphor in the presence of an alkalimetal ion.

In each example a sample consisting of 14.72 grams of trilactohafnicacid (prepared as described in Examples 1-9 from RGS hafniumoxychloride, z=0.019) was thoroughly ground with an agate mortar andpestle with K₂ CO₃ or Li₂ CO₃ (Alfa Products; Ultra Pure grade). Themole percent of the alkali carbonate flux, based on hafnium, was chosenas indicated below in Table III. The mixtures prepared were heated asdescribed above in Examples 1-9, except for the addition of a washingstep after firing to 1000° C. This step involved washing the charge with150 mL of distilled water for 1 hour. The solid was collected and driedfor 5 minute intervals at 20, 35 and 50% power in a 500W CEM model MDS81™ microwave oven. The procedure described above in Examples 1-9 wasthen completed.

X-ray diffraction analysis of the samples confirmed the presence ofmonoclinic hafnia. The presence of alkali metal ion in the phosphorpowder samples prepared in the presence of alkali carbonate flux wasconfirmed by atomic absorption analysis.

                  TABLE III                                                       ______________________________________                                        DM.sub.m                                                                      Example  M         m      Intensity (Ex. 1 = 100)                             ______________________________________                                         5       --        --     365                                                 10       K         0.2    520                                                 11       K         0.5    510                                                 12       K         2.0    545                                                 13       K         4.0    1005                                                14       Li         0.14  1005                                                ______________________________________                                    

A 140 to 275 percent increase in luminescence intensity relative toExample 5 is seen in the above examples containing alkali metal ion.

Referring back to Example 1, it is apparent that the hafnia phosphorsamples containing both zirconium in higher levels than found in opticalgrade hafnium sources and alkali metal ion exhibit luminescenceintensities ranging from >5 to >10 times those demonstrated by thehafnia phosphor prepared from an optical grade hafnium source.

Examples 15-18 Titanium Activated Phosphors (DTi_(x))

The purpose of presenting these investigations is to demonstrate theutility of titanium as an activator for the hafnia phosphors of thisinvention containing higher than optical grade concentrations ofzirconia. The titanium also shifts the maximum spectral emission band ofthe phosphor to visible wavelengths in the blue portion of the spectrum.

In each example a sample consisting of 14.72 grams of trilactohafnicacid (prepared as described above in Examples 1-9, z=0.019) wasthoroughly ground with varying portions of ammoniumbis(oxalato)-oxotitanium (IV), (NH₄)₂ TiO(C₂ O₄)₂ 2H₂ O, from JohnsonMatthey (99.998%). The mole percent titanium, based on hafnium, isindicated below in Table IV. The mixtures were heated and furtherexamined as in Examples 1-9.

X-ray diffraction analyses of Examples 17 and 18 each showed traces ofunreacted TiO₂. A small amount of hafnium titanate was detected as animpurity phase in Example 18.

The relative luminescence outputs of Examples 5 and 15-18 are set out inTable IV. Not only were the luminescence outputs greatly increased inExamples 15-18, but the luminescence band maximum shifted to 475 nm,thereby providing increased emissions of visible spectrum wavelengthsmore advantageous for intensifying screen applications.

                  TABLE IV                                                        ______________________________________                                        DTi.sub.x                                                                     Example      x      Intensity (Ex. 1 = 100)                                   ______________________________________                                         5           --      365                                                      15           0.02   5330                                                      16           0.05   4000                                                      17           0.10   2730                                                      18           0.25   1680                                                      ______________________________________                                    

From Table IV it is apparent that the inclusion of titanium in thehafnia phosphor samples containing higher than optical grade zirconiumconcentrations resulted in large increases in luminescence intensities.Thus, the titanium acted as an activator for the phosphor samples.

Examples 19-33 Preparation of Titanium Activated Phosphors in thePresence of Lithium Carbonate (DTi_(x) Li_(m))

The purpose of presenting these investigations is to demonstrate thatthe performance of hafnia host phosphors with an elevated zirconiumlevel (z=0.019) and containing titanium as an activator can be furtherimproved by Preparing the hafnia phosphor in the presence of an alkalimetal ion.

A sample consisting of 12.26 g of trilactohafnic acid (prepared as inExamples 1-9) was thoroughly ground with 0.1 g (5 mole percent, x=0.05)of TiO₂ (EM Chemicals; Optipur grade) and a selected amount of Li₂ CO₃(Alfa Products; Ultrapure grade). The mixtures were processed and testedsimilarly as in Examples 10-14. In Examples 21-23 the size of thetrilactohafnic acid sample was 13.00 grams with the titania increased to0.106 g to maintain the titanium at 5 mole percent (x=0.05).

The relative intensity of the titanium activated phosphor samples as afunction of the alkali metal flux employed is given in Table V.

                  TABLE V                                                         ______________________________________                                        DTi.sub.x M.sub.m                                                             Example      m      Intensity (Ex. 1 = 100)                                   ______________________________________                                        19           0      2520                                                      20           0.01   2210                                                      21           0.02   1000                                                      22           0.06   3380                                                      23           0.10   6370                                                      24           0.10   5960                                                      25           0.20   13500                                                     26           0.20   14000                                                     27           0.40   13700                                                     28           0.50   13300                                                     29           0.50   13500                                                     30           1.0    8695                                                      31           1.5    5610                                                      32           2.0    3155                                                      33           4.0    735                                                       ______________________________________                                    

Samples in which more than 10 mole percent (m=0.20) Li₂ CO₂ was addedrevealed the presence of lithium hafnate in the X-ray powder patterns.The amount of lithium hafnate formed in the samples increased with theLi₂ CO₃ amount. At 200 mole percent (m=4.0) Li₂ CO₃ added, lithiumhafnate is the primary phase.

From Table V it can be appreciated that values of m of from about 4×10⁻²(0.04) to 2.0 gave significantly improved results, with values of m offrom about 1×10⁻¹ (0.10) to 1.5 providing the highest luminescenceintensities observed in these comparisons.

In these comparisons it should be noted that Example 19 did not provideluminescence intensity as high as that reported in Table IV for Example16, even though both contained 5 mole percent titanium (x=0.05) andneither was prepared in the presence of an alkali metal flux. Thisdifference is attributed to the less efficient incorporation of thetitanium activator in Example 19 resulting from employing titania ratherthan a titanium carboxylate salt as a starting material.

Examples 34-43 Preparation of Titanium Activated Phosphors in thePresence of Lithium Sulfate (DTi_(x) Li_(m))

The purpose of presenting these investigations is to demonstrate thatthe proportions of lithium hafnate formed as a second phase can becontrolled and reduced by substituting another lithium salt for lithiumcarbonate.

The same procedures were employed as in Examples 19-33, except that forLi₂ CO₃ there was substituted Li₂ SO₄ (Aldrich anhydrous: 99.99%).

The relative intensity of the titanium activated phosphor samples as afunction of the lithium sulfate flux employed is given in Table VI. InTable VI the performance data from Table V is also represented forsamples prepared using lithium carbonate at the same concentrationlevels as the lithium sulfate.

                  TABLE VI                                                        ______________________________________                                        DTi.sub.x M.sub.m                                                             Li.sub.2 CO.sub.3 Li.sub.2 SO.sub.4                                           Example m      Intensity  Example                                                                              m     Intensity                              ______________________________________                                        20      0.01   2210       34     0.01  1545                                   21      0.02   1000       35     0.02  1545                                                             36     0.04  2105                                   22      0.06   3380       37     0.06  3605                                   23      0.10   6370       38     0.10  7645                                   24      0.10   5960                                                           25      0.20   13500      39     0.20  9115                                   26      0.20   14000                                                          28      0.50   13300      40     0.50  12400                                  30      1.0    8695       41     1.0   9820                                   32      2.0    3155       42     2.0   9330                                   33      4.0    735        43     4.0   9185                                   ______________________________________                                    

The most important advantage of employing lithium sulfate as a flux ascompared to lithium carbonate is that a reduced amount of the lithiumhafnate phase is produced. This results in significant improvements inphosphor luminescence when higher proportions of the lithium flux areemployed during phosphor formation. At lower, preferred fluxconcentrations the lithium carbonate flux yields higher luminescence.

Examples 44-47 Preparation of Phosphors in the Presence of Varied AlkaliMetal Ions

The purpose of presenting these investigations is to demonstrate thatall of the alkali metals significantly enhance phosphor luminescence.

Example 25 was repeated, except that 10 mole percent (m=0.2) of anotheralkali metal carbonate was substituted for lithium carbonate: Na₂ CO₃(0.265 g; EM Chemicals Suprapur Reagent), K₂ CO₃ (0.346 g; Alfa ProductsUltrapure grade), Rb₂ CO₃ (0.5774 g; AESAR 99.9%), or Cs₂ CO₃ (0.8146 g;AESAR 99.9%).

The luminescence intensities measured for the resulting samples are setout in Table VII.

                  TABLE VII                                                       ______________________________________                                        Example   Carbonate source                                                                           Intensity (Ex. 1 = 100)                                ______________________________________                                        19        None         2520                                                   25        Li.sub.2 CO.sub.3                                                                          13500                                                  44        Na.sub.2 CO.sub.3                                                                          10400                                                  45        K.sub.2 CO.sub.3                                                                           5400                                                   46        Rb.sub.2 CO.sub.3                                                                          3645                                                   47        Cs.sub.2 CO.sub.3                                                                          4840                                                   ______________________________________                                    

From Table VII it is apparent that all of the alkali metals areeffective to increase the luminescence of the hafnia phosphors preparedfrom sources having higher zirconium contents than found in opticalgrade sources of hafnium. From Table VII it is observed that the lowerthe atomic number alkali metals lithium, sodium, and potassium offer asignificant performance advantage over the heavier alkali metalsrubidium and cesium when equal starting concentrations are employed.

Examples 48-51 Preparation of Phosphors Using Varied Alkali MetalCompounds

The purpose of presenting these investigations is to demonstrate theutility of alkali metal compounds completed by moieties other thansulfate and carbonate.

Example 25 was repeated, except that one of the following lithiumsources was substituted for lithium carbonate: 0.2548 g Li₂ C₂ O₄ (10mole percent, m=0.2, Alfa Products reagent grade), 0.212 g LiCl (20 molepercent, m=0.2, Alfa Products anhydrous Ultrapure grade), 0.4343 g LiBr(20 mole percent, m=0.2, MCB anhydrous) or 0.21 g LiOH-H₂ O (20 molepercent, m=0.2, MCB reagent).

The luminescence intensities are given in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        Example   Lithium Cmpd.                                                                             Intensity (Ex. 1 = 100)                                 ______________________________________                                        19        None        2520                                                    48        Li.sub.2 C.sub.2 O.sub.4                                                                  12695                                                   49        LiCl        6730                                                    50        LiBr        9400                                                    51        LiOH:H.sub.2 O                                                                            13185                                                   ______________________________________                                    

From Table VIII it is apparent that all of the lithium compounds improvethe luminescence of the phosphor. While both lithium hydroxide andlithium oxalate produced significantly higher levels of luminescencethan the lithium halides, alkali carboxylates are clearly moreconvenient to handle than alkali hydroxides.

Examples 52-54 Enhancement of Phosphor Luminescence by a Combination ofTitanium and Alkali Metal Ion

The purpose of presenting these investigations is to demonstrate thesynergistic improvement of luminescence produced by the combination ofan alkali metal ion and the titanium activator.

EXAMPLE 52

A sample consisting of 13.475 g of trilactohafnic acid (prepared asdescribed in Examples 1-9) was thoroughly ground in an agate mortar andpestle with 0.2032 g Li₂ CO₃ (10 mole percent, m=0.2, Alfa ProductsUltrapure grade) and processed as in Examples 10-14.

EXAMPLE 53

Example 15 was repeated, except that 13.475 g of trilactohafnic acid wasused with 0.44 g of TiO₂ (2 mole percent, x=0.02, EM chemicals Optipurgrade).

EXAMPLE 54

Example 53 was repeated, except for the addition of 0.2032 g Li₂ CO₃ (10mole percent, m=0.2, Alfa Products Ultrapure grade) in the startingmixture.

The luminescence performances of Examples 5 and 52-54 are compared inTable IX.

                  TABLE IX                                                        ______________________________________                                        Example Additions       Intensity (Ex. 1 = 100)                               ______________________________________                                         5      none            365                                                   52      10 mole % Li.sub.2 CO.sub.3                                                                   1120                                                  53       2 mole % TiO.sub.2                                                                           5690                                                  54      10 mole % Li.sub.2 CO.sub.3 +                                                                 14600                                                          2 mole % TiO.sub.2                                                   ______________________________________                                    

From Table IX it is apparent that a disproportionately large increase inluminescence was realized by employing both the titanium activator andthe alkali metal ion. While each of the titanium and alkali metal aloneenhanced luminescence, a larger increase in luminescence was attainedwhen titanium and alkali metal ion were employed together than couldhave been predicted assuming the separate enhancements of luminescenceto be fully additive.

Examples 55-62 Phosphors Containing 5 Mole Percent or Less Titanium

The purpose of presenting these investigations is to demonstrate theenhancements in luminescence produced by the use as starting materialsof titanium at concentrations of 5 mole percent (x=0.05) and less,thereby presenting a better performance definition of the lower rangesof titanium concentrations.

Potassium tetraoxalatohafnate (IV) 5-hydrate was prepared as describedin Inorg. Syn., VIII, 42 (1966) using R.G.S. hafnium oxychloride 8hydrate (z=0.019). Upon drying at 70°-90° C. for 1-16 hours in aconvection oven, the product analyzed at closer to a 3-hydratecomposition and all subsequent use of this material was calculated asthe 3-hydrate. Fifteen grams of the material was thoroughly ground in anagate mortar and pestle with 0.03-5 mole percent of potassiumbis(oxalato)oxotitanate (IV) 2 hydrate (Alfa Products, recrystallizedfrom ethanol). The mixtures were placed in 20 mL alumina crucibles,covered with alumina lids, and then placed in 100 mL alumina crucibles,which were covered with alumina lids. The samples were heated in air to1000° C. for 2.5 hours, then cooled to room temperature. The resultingsolids were removed from the crucibles, broken into small pieces with analumina mortar and pestle and washed by stirring in 50 mL of distilledwater. The solids were then collected and dried in a convection oven at80° C. The charges were placed in 10 mL alumina crucibles with aluminalids and heated in air to 1300° C. for 2 hours, followed by cooling toroom temperature.

The luminescence intensities of the samples are set out in Table X.

                  TABLE X                                                         ______________________________________                                        Example   Mole percent Ti                                                                            Intensity (Ex. 1 = 100)                                ______________________________________                                         5        None         365                                                    55        0.03         5750                                                   56        0.3          6128                                                   57        1            9470                                                   58        2            10500                                                  59        3            8420                                                   60        3            9820                                                   61        4            8060                                                   62        5            9120                                                   ______________________________________                                    

From Table X it is apparent that even at the lowest concentrations oftitanium (DTi_(x) where x=3×10⁻⁴, Example 55) much higher levels ofluminescence are observed than in Example 5, which lacked titanium.While some of the enhancement in luminescence as compared to Example 5can be attributed to the presence of potassium, comparing luminescencevalues from Table III, in which potassium was introduced withouttitanium being present, it is apparent that a part of the luminescenceenhancement must be attributed to additional presence of the titanium.

Examples 63-68 Varied Levels of Zirconium in Phosphors Prepared in thePresence of Alkali Metal Ion

The purpose of presenting these investigations is to demonstrate theeffect of varied levels of zirconium in the hafnia host phosphor whenthe hafnia phosphor was prepared in the presence of alkali metal ion.

Two grades of potassium tetraoxalatohafnate (IV) 3-hydrate were preparedas in Example 55 from optical grade hafnium oxychloride 8 hydrate andR.G.S. hafnium oxychloride 8-hydrate. Potassium tetraoxalatozirconate3-hydrate was prepared as in Example 55 from R.G.S. zirconiumoxychloride 8-hydrate. A series of Hf_(1-z) Zr_(z) O₂ samples in which zwas varied from 2.76×10⁻⁴ to 6.84×10⁻² were prepared from mixtures ofthe above precursors. The powders were combined and ground in an agatemortar and pestle. The procedures of Examples 55-62 were employed, withthe addition of 10 mole percent K₂ CO₃ (Alfa Products Ultrapure grade)to each sample.

Luminescence intensities as a function of zirconium levels (z) are givenin Table XI.

                  TABLE XI                                                        ______________________________________                                        Example     z         Intensity (Ex. 1 = 100)                                 ______________________________________                                        63 (Control)                                                                              2.8 × 10.sup.-4                                                                   380                                                     64          4.3 × 10.sup.-4                                                                   165                                                     65          9.6 × 10.sup.-3                                                                   770                                                     66          1.9 × 10.sup.-2                                                                   520                                                     67          4.0 × 10.sup.-2                                                                   595                                                     68          6.0 × 10.sup.-2                                                                   610                                                     ______________________________________                                    

Note that Example 66 was identical to Example 10, except for employing adifferent final firing temperature, and the luminescence measured wasidentical.

Table XI demonstrates that hafnia prepared from optical grade sources asin Control Example 63 yields inferior luminescence as compared tosamples in which the zirconium content z is equal to at least 1×10⁻².Comparing Tables II and XI, it is apparent that the presence ofpotassium ion is responsible for a significant increase in luminescenceat zirconium levels equal to that in R.G.S. hafnia (z=0.019) and above.

Examples 69-72 Determinations of Alkali Metal Ion Incorporation inPhosphors Differing in Zirconium Levels

The purpose of presenting these investigations is to providequantitative determinations of alkali ion incorporation levels (y) inseveral phosphors satisfying the general relationship Hf_(1-z) Zr_(z)Ti_(x) M_(y) and having differing zirconium levels (z) satisfying therequirements of the invention.

Samples were prepared as in Examples 63-68, except for the furtheraddition of 0.2151 g of recrystallized potassium bis(oxalato)oxotitanate(IV) 2-hydrate (Alfa Products) to satisfy the ratio x=0.03.

Proportions of zirconium, titanium, and potassium ion in the completedphosphor samples were determined by atomic absorption analysis andinductively coupled plasma spectrometry. The luminescence of thephosphors together with their alkali ion content observed on analysis,y(obs), are reported in Table XII. The amounts of zirconium and titaniumpresent in the starting materials, z(calc) and x(calc), are compared inTable XII to the amounts of zirconium and titanium found on analysis,z(obs) and x(obs).

                                      TABLE XII                                   __________________________________________________________________________    Hf.sub.l-z Zr.sub.z Ti.sub.x M.sub.y                                             Intensity                                                                  Ex.                                                                              (Ex. 1 = 100)                                                                        z (calc)                                                                            z (obs)                                                                              x (calc)                                                                           x (obs)                                                                            y (obs)                                      __________________________________________________________________________    69 9820   4.3 × 10.sup.-4                                                               4.31 × 10.sup.-4                                                               0.03 0.022                                                                              0.022                                        70 9820   9.6 × 10.sup.-4                                                               8.79 × 10.sup.-4                                                               0.03 0.026                                                                              0.019                                        71 9820   1.9 × 10.sup.-2                                                               1.78 × 10.sup.-2                                                               0.03 0.031                                                                              0.025                                        72 9820   4.0 × 10.sup.-2                                                               3.87 × 10.sup.-2                                                               0.03 0.027                                                                              0.023                                        __________________________________________________________________________

Although all samples exhibited similar luminescence, when acorresponding phosphor was formed from optical grade hafnium startingmaterials [z(obs)=2.91×10⁻⁴ ], a significantly lower luminescence wasobserved.

Examples 73-100 Rare Earth Incorporations

The purpose of presenting these investigations is to demonstrate thecapability of lutetium to reduce afterglow in host phosphors havingtheir luminescence enhanced by the incorporation of titanium as anactivator and to demonstrate the unsatisfactory effects of the rareearth elements lanthanum, terbium, praseodymium, cerium, and dysprosium.

A hydrous hafnia precursor was prepared by a conventional preparationmethod. Suitable methods are those disclosed for preparing hydrouszirconia by M. Shibagaki, K. Takahasi, and M. Matsushita, Bull. Chem.Soc. Japan, 61, 3283 (1988) and A. Benedetti, G. Fagherazzi. and F.Pinna, J. Am. Ceram. Soc., 72, 467 (1989). Samples of 1.0 mole R.G.S.hafnium oxychloride (Hf_(1-z) Zr_(z), z=0.010) from Teledyne Wah ChangAlbany and 2.1 mole of sodium hydroxide pellets from Eastman KodakCompany were each dissolved in 1.0 liter of distilled water. Thesolutions were added simultaneously to a precipitation vessel with rapidstirring. The resulting gelatinous solid was :collected by vacuumfiltration and then dried using a rotary evaporator. The solid waswashed three times with 4 liters of distilled water. The collectedmaterial was then dried for 16 hours at 50° C. in a convection oven.

In each example a 0.0265 mole sample of precursor hydrous hafnia wasemployed. In all examples, except Example 73, the sample was treatedwith a measured mole percent of a rare earth ion source, either in theform of an aqueous solution or a solid. After addition the ingredientswere thoroughly mixed. In those instances in which a solution was usedthe samples were oven dried. Eight mole percent (based on hafnium)lithium carbonate (Aldrich, 99.997%) and 5 mole percent titanium dioxide(Aldrich 99.99%) were thoroughly ground and mixed into each sample. Eachsample was placed in a 10 mL alumina crucible and covered with analumina lid. The crucibles were heated to 1000° C. and maintained atthat temperature for 2.5 hours before being allowed to cool to roomtemperature. The samples were each washed in 150 mL of distilled waterfor one hour and then collected by vacuum filtration and dried for 5minute intervals at 20, 35 and 50 percent power in a microwave oven. Thesamples were then returned to their 10 mL crucibles in ambient air,covered, and heated to 1300° C. and maintained at that temperature for1.5 hours before being allowed to cool to room temperature. Theresulting powders were ground to give uniform phosphor powders.

To provide the best possible control for Purposes of comparison, severalExample 73 control samples were prepared. During each firing of rareearth doped phosphor samples one of the Example 73 control samples waspresent so that the control would experience exactly the same firingconditions as the rare earth doped phosphor being investigated. In thetables below, when the rare earth doped phosphors reported were not allfired simultaneously, the relative intensity and afterglow for thecontrol Example 73 was an average of the controls fired with that groupof phosphors. Relative intensities of the control samples ranged from13,460 to 14,520 (Ex. 1=100). To facilitate comparisons, the relativeintensity and relative afterglow characteristics of the control sample(or control sample average) reported in the tables below were each setat 100.

The afterflow characteristics of each Phosphor sample were determined byplacing the sample in a chamber and exposing it to X-radiation from atungsten target, beryllium window tube operated at 70 kVp and 10 mA,filtered with 0.5 mm Cu and 1 mm Al. The phosphor samples were preparedby placing the phosphor powder in aluminum planchets (2 mm deep×24 mmdiameter) at a coverage of about 1.1 g/cm². The emitted light wasdetected by a photomultiplier tube, the output current of which wasmeasured by a voltmeter across a load resistor. The voltmeter readingserved as an input to an x-y recorder which plotted the variation ofvoltage as a function of time. Constant irradiation by X-rays of eachsample produced a steady state reading on the voltmeter, which wasadjusted to 100% on the x-y recorder. The X-radiation was then shut off,and the decay of the light emitted by each sample was monitored. Theelapsed time required for the signal to decay to 1% of its steady statevalue was then read off the x-y plot. To facilitate comparison, controlExample 73, lacking a rare earth, was assigned a relative afterglowvalue of 100 percent, and the successive examples were assigned arelative afterglow value based on its relationship to control Example73.

Comparative Examples 73-77 Lanthanum Addition

The purpose of presenting these investigations is to demonstrate thatlanthanum was not found a satisfactory dopant to reduce afterglow.Lanthanum ions were provided in the form of La₂ (C₂ O₄)₃ 9-10H₂ Oprepared from La₂ O₃ (Molycorp 99.99%).

The relative luminescence outputs and afterglow values of the lanthanumdoped samples and the control lacking lanthanum addition are set out inTable XIII.

                  TABLE XIII                                                      ______________________________________                                        DTi.sub.x La.sub.w                                                                                    Relative Relative                                     Example     w           Intensity                                                                              Afterglow                                    ______________________________________                                        73a (Control)                                                                             0.00        100      100                                          74 (Comp. Ex.)                                                                            2.5 × 10.sup.-6                                                                     105      97                                           75 (Comp. Ex.)                                                                            5.0 × 10.sup.-5                                                                     105      93                                           76 (Comp. Ex.)                                                                            1.0 × 10.sup.-4                                                                     103      102                                          77 (Comp. Ex.)                                                                            5.0 × 10.sup.-4                                                                     103      88                                           ______________________________________                                    

Although lanthanum had no measurable adverse effect on prompt emissionintensity and therefore could be easily tolerated in the phosphor, ithad no clearly beneficial effect on afterglow. In Example 76 at aconcentration of w=1×10⁻⁴ no measurable reduction in afterglow wasobserved. By comparison of the results reported in Table XIII with thosereported for lutetium doped phosphors according to the inventionreported in Table XVIII below. It is apparent that the performance oflutetium as a dopant to reduce afterglow is significantly superior tothat of lanthanum.

Comparative Examples 78-80 Terbium Addition

The purpose of presenting these investigations is to demonstrate thatterbium was not found a satisfactory dopant to reduce afterglow. Terbiumions were provided in the form of an aqueous solution of Tb(NO₃)₃ 6H₂ O(Alfa 99.9%).

The relative luminescence outputs and afterglow values of the terbiumdoped samples and the control lacking rare earth addition are set out inTable XIV.

                  TABLE XIV                                                       ______________________________________                                        DTi.sub.x Tb.sub.w                                                                                    Relative Relative                                     Example     w           Intensity                                                                              Afterglow                                    ______________________________________                                        73b (Control)                                                                             0.00        100      100                                          78 (Comp. Ex.)                                                                            5.0 × 10.sup.-7                                                                     88       108                                          79 (Comp. Ex.)                                                                            1.0 × 10.sup.-6                                                                     82       95                                           80 (Comp. Ex.)                                                                            1.0 × 10.sup.-5                                                                     57       70                                           ______________________________________                                    

Terbium produced only a very modest reduction in afterglow at theexpense of very substantial reductions in prompt luminescenceintensities. Notice that in every instance the percentage reduction inrelative intensity was greater than the reduction in relative afterglow.By comparison of the results reported in Table XIV with those reportedfor lutetium doped phosphors according to the invention reported inTable XVIII below, it is apparent that the performance of lutetium as adopant to reduce afterglow is significantly superior to that of terbium.

Comparative Examples 81-86 Praseodymium Addition

The purpose of presenting these investigations is to demonstrate thatpraseodymium was not found a satisfactory dopant to reduce afterglow.Praseodymium ions were provided in the form of an aqueous solution ofPr(NO₃)₃ 6H₂ O (Reaction 99.99%, Rare Earth Products) in Examples 81-83,with the nitrate solid being used in the remaining examples.

The relative luminescence outputs and afterglow values of thepraseodymium doped samples and the control lacking rare earth additionare set out in Table XV.

                  TABLE XV                                                        ______________________________________                                        DTi.sub.x Pr.sub.w                                                                                    Relative Relative                                     Example     w           Intensity                                                                              Afterglow                                    ______________________________________                                        73c (Control)                                                                             0.00        100      100                                          81 (Comp. Ex.)                                                                            5.0 × 10.sup.-7                                                                     101      108                                          82 (Comp. Ex.)                                                                            2.5 × 10.sup.-6                                                                     95       97                                           83 (Comp. Ex.)                                                                            5.0 × 10.sup.-5                                                                     69       89                                           84 (Comp. Ex.)                                                                            1.0 × 10.sup.-4                                                                     77       82                                           85 (Comp. Ex.)                                                                            2.5 × 10.sup.-4                                                                     57       68                                           86 (Comp. Ex.)                                                                            5.0 × 10.sup.-4                                                                     53       76                                           ______________________________________                                    

Praseodymium produced only a very modest reduction in afterglow at theexpense of very substantial reductions in prompt luminescenceintensities. Notice that in every instance the percentage reduction inrelative intensity was greater than the reduction in relative afterglow.By comparison of the results reported in Table XIV with those reportedfor lutetium doped phosphors according to the invention reported inTable XVII below, it is apparent that the performance of lutetium as adopant to reduce afterglow is significantly superior to that ofpraseodymium.

Comparative Examples 87-90 Cerium Addition

The purpose of presenting these investigations is to demonstrate thatcerium was not found a satisfactory dopant to reduce afterglow. Ceriumions were provided in the form of Ce₂ (C₂ O₄)₃ 9H₂ O (Reaction 99.99%).

The relative luminescence outputs and afterglow values of the ceriumdoped samples and the control lacking rare earth addition are set out inTable XVI.

                  TABLE XVI                                                       ______________________________________                                        DTi.sub.x Ce.sub.w                                                                                    Relative Relative                                     Example     w           Intensity                                                                              Afterglow                                    ______________________________________                                        73d (Control)                                                                             0.00        100      100                                          87 (Comp. Ex.)                                                                            5.0 × 10.sup.-5                                                                     94       113                                          88 (Comp. Ex.)                                                                            1.0 × 10.sup.-4                                                                     94       107                                          89 (Comp. Ex.)                                                                            2.5 × 10.sup.-4                                                                     84       110                                          90 (Comp. Ex.)                                                                            5.0 × 10.sup.-4                                                                     77       113                                          ______________________________________                                    

Not only did cerium prove ineffective to reduce afterglow, it producedmeasurable increases in afterglow while also decreasing the intensity ofprompt luminescence. Thus, cerium had a uniformly detrimental effect onphosphor performance.

Comparative Examples 91-95 Dysprosium Addition

The purpose of presenting these investigations is to demonstrate thatdysprosium was not found a satisfactory dopant to reduce afterglow.Dysprosium ions were provided in the form of an aqueous solution ofDy(NO₃)₃ 5H₂ O (Alfa, 99.9%) in Examples 91-93, with the nitrate solid bin the remaining examples.

The relative luminescence outputs and afterglow values of the dysprosiumdoped samples and the control lacking rare earth addition are set out inTable XVII.

                  TABLE XVII                                                      ______________________________________                                        DTi.sub.x Dy.sub.w                                                                                    Relative Relative                                     Example     w           Intensity                                                                              Afterglow                                    ______________________________________                                        73e (Control)                                                                             0.00        100      100                                          91 (Comp. Ex.)                                                                            1.0 × 10.sup.-6                                                                     92       300                                          92 (Comp. Ex.)                                                                            2.5 × 10.sup.-6                                                                     84       284                                          93 (Comp. Ex.)                                                                            5.0 × 10.sup.-5                                                                     73       460                                          94 (Comp. Ex.)                                                                            1.0 × 10.sup.-4                                                                     65       500                                          95 (Comp. Ex.)                                                                            2.5 × 10.sup.-4                                                                     49       492                                          ______________________________________                                    

Dysprosium increased afterglow from 3 to 5 times while also decreasingthe intensity of prompt luminescence. Thus, dysprosium had a uniformlydetrimental effect on phosphor performance.

Examples 96-100 Lutetium Addition

The purpose of presenting these investigations is to demonstrate thathost phosphors satisfying the requirements of this invention havingtheir intensity of prompt emission increased by titanium incorporationhave their afterglow significantly reduced by the incorporation oflutetium. Lutetium ions were provided in the form of aqueous solutionsof LuCl₃ 6H₂ O (Alfa, 99.9%).

The relative luminescence outputs and afterglow values Of the lutetiumdoped samples and the control lacking rare earth addition are set out inTable XVIII.

                  TABLE XVIII                                                     ______________________________________                                        DTi.sub.x Lu.sub.w                                                                                    Relative Relative                                     Example    w            Intensity                                                                              Afterglow                                    ______________________________________                                         73f (Control)                                                                           0.00         100      100                                           96 (Comp. Ex.)                                                                          2.5 × 10.sup.-6                                                                      98       111                                           97        5.0 × 10.sup.-5                                                                      96       56                                            98        1.0 × 10.sup.-4                                                                      100      44                                            99        2.5 × 10.sup.-4                                                                      94       31                                           100        5.0 × 10.sup.-4                                                                      87       24                                           ______________________________________                                    

From Table XVIII it is apparent that a significant reduction inafterglow with little, if any loss of prompt emission intensity can berealized when Lu is present in concentrations of as low as ten parts permillion (w=1×10⁻⁵). On the other hand, from limited reduction in promptemission intensity exhibited by Ex. 100, can be reasonably relied uponto reduce afterglow while allowing acceptable prompt emissionintensities at concentrations of lutetium of up to w=1×10⁻³ or evenhigher. From Table XVIII it is apparent that a preferred Luconcentration is achieved when w is in the range of from 5×10⁻⁵ to3×10⁻⁴.

Other rare earths have been observed to be effective in reducingafterglow in titanium activated zirconium and hafnium containingphosphors and are the subject matter of concurrently filed, commonlyassigned patent applications.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A screen comprised ofa support and a fluorescentlayer containing a phosphor capable of absorbing X-radiation andemitting longer wavelength electromagnetic radiation comprised ofmonoclinic crystals of a titanium activated hafnium dioxide phosphorhost, characterized in that the monoclinic crystals consist essentiallyof oxygen and combined elements satisfying the relationship

    Hf.sub.1-z Zr.sub.z Ti.sub.x M.sub.y Lu.sub.w

wherein M represents at least one alkali metal; w is in the range offrom 1×10⁻⁵ to 1×10⁻³ ; x is in the range of from 3×10⁻⁴ to 1.0; y is upto 1; and z is up to 0.3, the phosphor exhibiting a reduced afterglowwhen excited by X-radiation as compared to the phosphor as defined aboveabsent lutetium.
 2. A screen according to claim 1 further characterizedin that z is at least 4×10⁻⁴.
 3. A screen according to claim 2 furthercharacterized in that z is in the range of from 1×10⁻³ to 0.2.
 4. Ascreen according to claim 3 further characterized in that z is in therange of from 2×10⁻³ to 0.1.
 5. A screen according to claim 1 furthercharacterized in that x is in the range of from 3×10⁻⁴ to 0.5.
 6. Ascreen according to claim 1 further characterized in that x is in therange of from 3×10⁻⁴ to 0.25.
 7. A screen according to claim 1 furthercharacterized in that y is at least 1×10⁻⁴.
 8. A screen according toclaim 7 further characterized in that y is in the range of from 1×10⁻⁴to 0.2.
 9. A screen according to claim 8 further characterized in that yis at least 8×10⁻⁴ and the alkali metal ions include at least one oflithium, sodium, and potassium.
 10. A screen according to claim 1further characterized in that w is in the range of from 5×10⁻⁵ to3×10⁻⁴.